Its not what you put into your body that makes you unclean...but what comes out of your mouth. Thats what the Bible says and thats what I believe. So, if smoking weed or drinking alcohol causes you to sin then maybe you should re-evaluate its place in your life. If it does not cause you to sin, then maybe evaluate what impact it has on your relationship with God. If it hinders your relationship...maybe you ought to consider that. Maybe it improves your quality of life and relationship with others.
There things that some consider sin and others do not. The bible says this. Specifically says this in regards to putting substances in your body actually...it says that if your brothers conscience tells him that eating a certain food is sin....then guess what? To that person it IS sin! BUT...if another does not have a conviction regarding that food then it is NOT sin.
Its very simple but Christians make it a stumbling block when they put their own personal convictions on another.
To the alcoholic drinking is a sin because of what it does to him. To another, maybe a glass of wine with dinner is a blessing! But the bible says we shouldnt use our freedom to make another stumble so if im around the alcoholic, I wouldnt want to tempt him by having a drink in front of him. Its simple.
Be true to your convictions and stop judging others for living in the freedom Christ has givin them.
A dope smoking Christian is a stumbling block both to his brothers in Christ, and also to unbelievers:
Rom 14:20 Do not, for the sake of food, destroy the work of God. Everything is indeed clean, but it is wrong for anyone to make another stumble by what he eats.
Rom 14:21
It is good not to eat meat or drink wine or do anything that causes your brother to stumble.
Rom 14:22 The faith that you have, keep between yourself and God. Blessed is the one who has no reason to pass judgment on himself for what he approves.
Rom 14:23
But whoever has doubts is condemned if he eats, because the eating is not from faith.
For whatever does not proceed from faith is sin
Our freedom in Christ is never to be used to sin:
Galatians 5:13
For you were called to freedom, brethren; only do not turn your freedom into an opportunity for the flesh, but through love serve one another.
1 Peter 2:16
Act as free men, and do not use your freedom as a covering for evil, but use it as bondslaves of God.
An American Christian who smokes dope is sinning, because it is outlawed on a federal level.
Romans 13:1
Everyone must submit himself to the governing authorities, for there is no authority except that which God has established. The authorities that exist have been established by God
Marijuana is an intoxicant and we are commanded to be sober minded:
Rom_12:3 For by the grace given to me I say to everyone among you not to think of himself more highly than he ought to think, but to think with sober judgment, each according to the measure of faith that God has assigned.
1Th_5:6 So then let us not sleep, as others do, but let us keep awake and be sober.
1Th_5:8 But since we belong to the day, let us be sober, having put on the breastplate of faith and love, and for a helmet the hope of salvation.
1Ti_3:2 Therefore an overseer must be above reproach, the husband of one wife, sober-minded, self-controlled, respectable, hospitable, able to teach,
1Ti_3:11 Their wives likewise must be dignified, not slanderers, but sober-minded, faithful in all things.
2Ti_4:5 As for you, always be sober-minded, endure suffering, do the work of an evangelist, fulfill your ministry.
Tit_2:2 Older men are to be sober-minded, dignified, self-controlled, sound in faith, in love, and in steadfastness.
1Pe_1:13 Therefore, preparing your minds for action, and being sober-minded, set your hope fully on the grace that will be brought to you at the revelation of Jesus Christ.
1Pe_4:7 The end of all things is at hand; therefore be self-controlled and sober-minded for the sake of your prayers.
1Pe_5:8 Be sober-minded; be watchful. Your adversary the devil prowls around like a roaring lion, seeking someone to devour.
But if you dare use a little of Gods green herb, which God Himself gave for "the service of man", to help you end your day and relax your body and nerve
It doesn't serve man to violate Gods commands and sin by becoming intoxicated.
In the situation you are describing, that person is in a state of spiritual paralysis because he is leaning upon marijuana instead of the Lord, and it has become an idol in his life.